[PATCH 34/59] x86: Fix THREAD_GSCOPE_RESET_FLAG build on clang

[Adhemerval Zanella Netto]

On 19/10/25 12:21, Uros Bizjak wrote:
> 
> 
> On 10/17/25 21:13, Adhemerval Zanella wrote:
>> clang does not support __seg_fs in asm constraint.
>> ---
>>   sysdeps/x86_64/nptl/tls.h | 21 +++++++++++++++++----
>>   1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
>> index abfeb88054..2c4b9aaa80 100644
>> --- a/sysdeps/x86_64/nptl/tls.h
>> +++ b/sysdeps/x86_64/nptl/tls.h
>> @@ -198,13 +198,26 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
>>   # define THREAD_GSCOPE_FLAG_UNUSED 0
>>   # define THREAD_GSCOPE_FLAG_USED   1
>>   # define THREAD_GSCOPE_FLAG_WAIT   2
>> +
>> +/* clang does not support __seg_fs in asm constraint.  */
>> +# ifdef __clang__
>> +#  define XCHGL_GSCOPE(__r)                              \
>> +   asm volatile ("xchgl %0, %%fs:%P1"                          \
>> +                 : "=r" (__r)                              \
>> +                 : "i" (offsetof (struct pthread, header.gscope_flag)),          \
>> +           "0" (THREAD_GSCOPE_FLAG_UNUSED))
> 
> This needs a "memory" clobber, because this asm hides memory access
> from the compiler. Please see the "memory" special clobber argument
> section in [1].
> 
> Also, please use the "c" operand modifier instead of "P". Clang allows
> "c" with "i" constraint and also checks that operand fits "c".

Ack.

> 
> Clang accepts __seg_fs prefixed addresses, it just "forgets" to emit %fs: prefix in the generated code. While this could be considered a bug with clang, the simplest solution is to just conditionally put "%%fs:" prefix in the asm template for clang.

We also need to use a 'i' constraint, clang does not accept 'm' witha 
'((struct pthread __seg_fs *)0)->header.gscope_flag'. 

> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Clobbers-and-Scratch-Registers
> 
> Uros.